1. Field of the Invention
The present invention relates generally to optical switches, and particularly to liquid crystal optical switches.
2. Technical Background
Twisted (TN) and Supertwist (STN) nematic liquid crystal devices are well known and can be found in numerous applications. The most prevalent use of TN/STN devices is in the area of displays; however, these devices have been proposed for optical communications applications.
The nematic liquid crystal cells include alignment layers that cause the liquid crystal molecules to form a 90xc2x0 helix. The helix functions as a waveguide. When no voltage is applied a polarized light signal is rotated by approximately 90xc2x0 by adiabatic following. When power is applied to the cell, the helical alignment of the liquid crystal molecules is disrupted and the polarized light signal passes through the cell without being rotated.
In order for optical switches or wavelength selective cross-connect devices to be feasible, they must exhibit low intra-channel crosstalk. The maximum amount of cross-talk that can be tolerated is about xe2x88x9235 dB. System designers are specifying systems having cross-talk that is less than xe2x88x9240 dB. The measure of cross-talk in an LC cell sandwiched between parallel polarizing plates is the transmissivity.
The transmissivity and its reciprocal, the extinction ratio, is directly related to the degree of rotation provided by the helix in the LC cell. An LC cell that perfectly rotates a polarized light signal by 90xc2x0 would have zero transmissivity and an infinite extinction ratio. This is known as the xe2x80x9cminimum conditionxe2x80x9d and will be discussed in more detail below. However, for all practical purposes, cells having a perfect 90xc2x0 helix do not exist. Thus, as discussed above, LC cells rotate a polarized light signal an amount approximately equal to 90xc2x0. When an orthogonally polarized light signal, for example, is rotated by the LC cell, the majority of the signal becomes a parallel polarized signal. However, because the rotation isn""t a perfect 90xc2x0, an orthogonal component remains. The orthogonal component is transmitted by the cell and is intra-channel cross-talk in the communications channel. Obviously, the results are similar when the input signal is a parallel polarized signal; a parallel component will remain.
In one approach that has been considered, a wedged-shaped nematic liquid crystal cell was employed in an optical switch. The switch included a wedge-shaped cell which was disposed perpendicular to the light beam. During use, the effective thickness of the cell was varied to obtain a minimum condition by sliding the wedge-shaped cell along a direction perpendicular to the beam. This approach has serious drawbacks. Since the minimum condition (see equation (2) below) is wavelength dependent, a different thickness is required for the minimum condition for each wavelength channel. The task of designing a multi-cell liquid crystal array wherein each cell has a different thickness or wedge shape is impractical. This is exacerbated by the need for a different mechanical actuator for each cell. Because of these factors, this design is limited to a few wavelengths at most. Another drawback is reliability of the design. It uses mechanical movement of the cell for tuning a cell into minimum condition. Moving parts experience fatigue and ultimately fail.
Thus, a need exists for an optical switch or wavelength selective cross-connect (WSXC) having an array of LC cells that are individually and dynamically tunable to provide an acceptable level of cross-talk. A need exists for an LC device that can be tuned without moving the cells or the light beam itself. In addition, a need exists for a device that can support many wavelength channels.
The present invention addresses the needs discussed above. A tunable liquid crystal switch that achieves an intra-channel crosstalk of less than xe2x88x9240 dB using only electronic compensation is disclosed. A cross-talk less than xe2x88x9250 dB is provided by combining coarse temperature tuning and electronic compensation. This is acheived by designing the thickness of the liquid crystal device to cause a minimum condition to occur at a maximum operating wavelength and a maximum temperature. Thus, the liquid crystal device is tunable over all operating wavelengths and temperatures using electronic compensation.
One aspect of the present invention is an optical device for directing a light signal. The optical device includes a liquid crystal element for modulating the light signal, wherein the liquid crystal element is characterized by an extinction ratio when in an off-state. The optical device also includes a voltage controller connected to the liquid crystal element, wherein the voltage controller supplies a bias voltage to the liquid crystal element in the off-state to drive the extinction ratio toward a minimum condition.
In another aspect, the present invention includes a method of directing a light signal in an optical device. The optical device includes a liquid crystal element for switching the light signal. The liquid crystal element is characterized by an extinction ratio when in an off-state. The method includes the steps of providing a voltage controller connected to the liquid crystal element to supply the liquid crystal element with bias voltages, and the step of supplying a bias voltage to the liquid crystal element in the off-state to drive the extinction ratio toward a minimum condition.
In another aspect, the present invention includes a method of fabricating an optical device for directing a light signal. The method includes the step of providing a liquid crystal element for switching the light signal, the liquid crystal element having a thickness xe2x80x9cdxe2x80x9d which causes a minimum condition of an extinction ratio to occur at a longest operating wavelength of the light signal and at a temperature greater than a maximum operating temperature of the optical device. The method also includes the step of providing a voltage controller connected to the liquid crystal element, wherein the voltage controller supplies a bias voltage to the liquid crystal element in the off-state to drive the extinction ratio toward the minimum condition for operating wavelengths shorter than the longest operating wavelength, and for temperatures lower than the maximum operating temperature.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.